This invention relates to a DC to frequency conversion circuit and particularly to such a circuit which provides an output which is an integrated function of the input.
During the course of measurement procedures there is frequently provided a DC value wherein the magnitude is representative of the variable being measured. In order to provide a digital count of this variable, it is necessary to convert the DC value into a pulse signal having a fixed average frequency. The pulses can then be counted and the count will provide the desired indication. Such requirements are frequently encountered in telemetry systems where the sensor provides a DC level output and the central station must provide a digital readout on a digital counter. In most cases, digital equipment is utilized for providing the conversion from DC to a pulse signal for digital counting. However, as is generally recognized, digital equipment is exceedingly complex and expensive and is hard to service as well as costly to replace. Analog circuits can also be used for this purpose, such as a voltage controlled oscillator where the DC level is converted into a pulse frequency output. However, such analog devices frequently require numerous components to form and shape the desired pulse output and is generally not accurate enough for most applications.
By way of example, one application which finds use of a DC to frequency conversion circuit is in connection with a fuel flow system for aircraft use. Such fuel flow systems typically include a turbine flow transmitter which is placed in the fuel line and provides an output frequency which is a function of the volume of fluid passing through the transmitter. The frequency is subsequently converted into a DC level representing the flow rate which is displayed on an indicator. However, if the total amount of fuel consumed is to be indicated on a digital counter, it is necessary to convert the DC level into a pulse signal. Since the pulse signal is to be representative of the amount of fuel consumed, it will be necessary for the DC to frequency converter to integrate the value while performing the conversion. Thus, the DC level represents a flow rate as a function of time, and the digital output must represent amount of fuel consumed, which is an integral of the flow rate.